Topic: 750f chain rubbing sprocket cover

[seven50four]

Hi there!
New member here.  This forum turns up a lot in my various cb750 google searches and I'm thankful for the valuable information I've always found.  Anyway, I'm the proud new owner of a 1978 cb750f.  The original motor was gone, and it had a 750K motor (number CB750E2708152) on it with second gear bad.  I've installed what I've been told is a '78 cb750k motor that came as part of the deal.  That motor number is CB750E3028368.  The curved aluminum cover that hides the drive sprocket on the engine came with the bike, but was off when I bought it.  I put it on, but the outside of the chain (a 530) rubs the inside of the cover.  I read that the 750f and 750k motors were basically the same other than valve size and compression.  The cover does not appear to have ever been black, so I don't think it was from the 750f motor.  Does anyone have any ideas on what's going on?  Any help is greatly appreciated.
-Dan

[bollingball]

This is very odd it should have a 630 chain that is even wider. check to make sure PO did not put any shims behind the sprocket it should float a couple of mm on the shaft. also look at the rear wheel for any changes that would have kicked the chain to the out side.


                                             Ken

[CycleRanger]

Check the number of teeth on the sprockets. They may not be stock.

Oh, and post some pictures. We guess much better with photos! 

[Really?]

10mm or 20mm offset sprocket?

Does the sprocket look like the offset one here?  http://www.cyclexchange.net/Chain%20Sprocket%20Page.htm

[madmtnmotors]

This is very odd it should have a 630 chain that is even wider. check to make sure PO did not put any shims behind the sprocket it should float a couple of mm on the shaft. also look at the rear wheel for any changes that would have kicked the chain to the out side.


                                             Ken

The 630 and 530 chains have a different PITCH (distance between the roller pins as measured along the length of the chain) but are the same width:

http://www.z1enterprises.com/reference/chaintypesize.aspx

An o-ring chain, on the other hand, is wider than a standard chain.

[scottly]

The 77/78 motors had output shafts that stuck out 10mm further than the earlier motors. If the 530 sprocket is offset 10mm, that might cause interference with the cover. (The offset front sprockets can be used when fitting an earlier motor into a K7/K8 frame.)

[seven50four]

[img]Thanks for the input everyone!  Every bit of it was quite helpful to this odd mystery. and like some kind of mystery, we can now piece this together.  I have to apologize for not mentioning a few details before.  The original owner told me that he installed the motor that was on the bike and that he got it off a '71 parts bike.  However, I checked the motor number and it's a '77 motor as I stated.  The next detail that I should have mentioned was that the sprocket on that motor appeared that it had been offset on both sides, but the offset on one side looks to have been ground off to make it flat.  It had a little side to side slop on the shaft.  I used the sprocket that came with the extra motor that i installed, which had offsets on both sides and no slop, since it appeared to not have been tampered with.  Both were 18 tooth 530 sprockets with the two threaded holes on either side of the center which I now believe to be pre-77 stock sprockets.  The chain is a 530 o-ring chain.  I found a sprocket site with some dimensions.  The stock 630 front sprocket on 77-78 cb750s is 10.7mm wide with an offset on only one side and no threaded holes, while the stock 530 sprocket used up to '76 is 12.7mm wide with offset on both sides and the 2 threaded holes.  My conclusion is that the first motor on my bike came out of the '71 bike, and as scottly stated, the 77 motor had the bigger offset on the shaft, so someone had to "customize" the '71sprocket by grinding  the offset off and facing that side in to work in the '71 bike.  The other sprocket was also from a pre-77 bike which explains the lack of side to side slop.
   I called jt sprockets and got the part number of a 17tooth 530 front sprocket that has the correct 10.7 width and spline dimensions.  (530 so I don't have to replace the rear sprocket or chain)  They were very helpful.  It turned out to be an '82 CB650 front sprocket.  I doubt that will help the cover-rubbing issue, but will at least be the right style of sprocket (with a little side to side play as bollingball stated is normal).  I also removed the rear wheel and swingarm, measured the swingarm and frame, reassembled and determined there are no serious defects and the rear wheel is in alignment.  As far as the rubbing goes, could it be 1.) the sprocket cover leaves too small an opening because it's from a pre-77 bike before the 10mm further out  output shaft? or 2.) an O-ring chain is too wide to fit past the cover?  I apologize for the very long post.

[madmtnmotors]

O-ring chains BARELY fit. My o-ring chain has rubbed the lower engine case ever so slightly, leaving some scoring on the outside of the lower case, not bad though.